1. Field of the Invention
The present invention relates to a method of forming a photo diode on a semiconductor wafer, and more particularly, to a method of forming a photo diode with dummy active areas.
2. Description of the Prior Art
The photo diode is a bi-electrode semiconductor device comprising a photo-conductivity cell and a junction diode and is typically used in the design and layout of optical products such as cameras and scanners. When not lit, the leakage current represents the noise but when lit the leakage current represents the signal. The photo diode manages signal information utilizing the signal-to-noise ratio. By increasing the intensity of the ratio of the signal to noise, the contrast of the signal and the quality of the photo diode are improved.
Please refer to FIG. 1 and FIG. 2. FIG. 1 is a schematic diagram of the structure of the prior art photo diode 10. FIG. 2 is a schematic diagram of the processing of the field oxide 16 on the photo diode 10. The formation of the prior art photo diode 10 is performed on the substrate 10 of a semiconductor wafer. The photo diode 10 comprises a photo sensor area 12 positioned on a predetermined area of the substrate 14, and a field oxide 16 positioned on the substrate 14 surrounding the photo sensor area 12. The field oxide 16 acts as dielectric insulating material to prevent short circuiting secondary to contact between the photo sensor area and other units.
In the formation of the prior art photo diode 10, a pad oxide layer 17 and a silicon nitride layer 18 are deposited in order on the substrate 14 and are used as a mask for the local oxidation of silicon to define an active area for the photo sensor area 12.
When the semiconductor wafer undergoes thermal oxidation in a furnace, the field oxide layer 16 is generated only on the portion of substrate 14 not covered by the silicon nitride layer 18 due to the fact that moisture and oxygen do not penetrate the silicon nitride layer 18 and substrate 14 well.
However, moisture and oxygen can still reach the substrate 14 immediately surrounding the silicon nitride layer 18 by spreading horizontally to cause oxidation. This causes a bird's beak profile on the field oxide layer 16, as shown in FIG. 2 and subsequent stress between the substrate 14 and the field oxide layer 16. As the thickness of the field oxide layer 16 increases, so does the stress between the substrate 14 and the field oxide 16. This leads to increase of leakage current in the PN junction of the photo sensor area 12 and noise of the photo diode 10.